reliance requires login through a web page. it should work fine considering you have a browser to login. airtel/bsnl does not. so they are safe. some other isps use software which are not going to work on an android tablet.
I guess most routers have to be initially setup through a LAN connection(wifi password and all), but after that you should be fine. but if you buy one with WPS support it should work directly I guess if the tablet too supports it.